This invention relates to the surface cleaning of copper base alloys and provides a rapid and effective two-stage process for removing surface oxide deposits formed on the metal, as during annealing treatments. In the case of copper alloys containing aluminum and other readily oxidizable metals such as iron, cobalt, nickel, zinc, or silicon, a resistant metal oxide surface film is formed during annealing treatments at elevated temperatures, applied for a period of time sufficient to relieve strains imposed during mechanical operations, as in the rolling of metal ingots to elongated strip of reduced thickness or for the fabrication of metal parts and articles. Such surface coatings of oxide are produced even when the annealing is carried out in the presence of a protective atmosphere, such as may be prepared by the partial combustion of a hydrocarbon, because the active metals present in the alloy will react with oxygen that is present in the free state or as moisture or as an oxide of carbon to form refractory oxides. When various metal oxides are thus formed, they often tend to combine with each other to form complex metal oxides, such as spinels, of refractory nature, which at times are extremely resistant to removal by conventional cleaning methods. Unless such surface oxide films are thoroughly removed, however, subsequent surface finishing treatments, such as soldering, electroplating, or pressure bonding with other metals, will generally yield unsatisfactory results because of the resulting poor adhesion.
While numerous proposals have been made in the past for single and multi-stage cleaning treatments, they have generally proved ineffective for the successful cleaning of copper alloys containing aluminum and other metals tending to form refractory oxides in thick surface layers. Although some of these procedures may, if prolonged, finally result in the removal of such layers, an undesirable extent of pitting and etching of the metal surface is found to occur simultaneously during such extended treatments, which may render them unsuitable for the intended purpose. Such unsatisfactory results characterize attempts to remove thick refractory oxide layers from various copper alloys containing aluminum and other active metals by single treatments with aqueous pickling solutions, such as aqueous solutions of sulfuric acid, sulfuric acid and alkali dichromate, sulfuric acid and ferric sulfate, as described on Pages 308-309 of "The Chemical Formulary", Vol. IX, 1951, Chemical Publishing Co., Inc., Brooklyn, N.Y.
Accordingly, it is the principal object of this invention to provide a rapid and effective treatment for the removal of refractory complex metal oxide films, even of unusually high thickness, from the surface of copper base alloys.
A further object is to provide a process that is readily applied and is effective for such removal without causing harmful surface etching or pitting of the metal.
Other objectives and advantages will be apparent from the following description.